Field of the Invention
The technical field of this disclosure is Micro Electro Mechanical Systems (MEMS), particularly, MEMS scanning micromirrors.
Related Art
MEMS scanning micromirrors have been developed for the display of visual information. The MEMS scanning micromirror oscillates in one or two dimensions and a laser or other light beam reflects from the mirror surface. Varying the angle and timing of the beam incident on the mirror surface generates a visual image on a screen or other surface, such as a two dimensional display matrix. Different numbers of MEMS scanning micromirrors and lasers are used to produce images of different detail and colors. Exemplary uses for the MEMS scanning micromirrors are video projection (e.g. in head up displays for automotive applications or for pico-projection in mobile phones), optical coherence tomography, and laser Doppler vibrometry.
U.S. Pat. No. 6,392,220 (B1) discloses a monolithically fabricated micromachined structure that couples a reference frame to a dynamic plate. In an embodiment a torsional scanner is arranged in a frame by torsion bars. Therein a respective tether is attached to each of the torsion bars. The tether includes connection rods each of which is coupled by a spring to the frame by flexible rods.
As the torsion bar rotates, the connection rods move and the flexible rods are pulled thereby compressing corrugations in the spring. Once the corrugations contact each other the tether becomes essentially inelastic and further rotation of a section of the torsion bar is impeded similar to the tethers causing the torsional spring constant to exhibit an abrupt change of value.
By attaching appendages or tethers to one or both of the torsion bars, the length of the torsion bar is effectively shortened at pre-established angles as the plate rotates about the axis thereby creating a marked change in the torsional spring constant of the torsional scanner at each such angle. In this way it is possible to provide the torsional scanner with a multi-segmented restoring torque curve that increases the electrostatic stability of the torsional scanner without distorting the mirror surface. Accordingly, the torsion bars, which also serve as vertical support beams, have a significant influence on the torsional stiffness with which the plate is arranged in the frame.
JP2008197140 discloses an optical scanner comprising a mirror rotatable arranged by torsion beams that are connected at a first end to a frame. The optical scanner further comprises a pair of cantilevers arranged at each side of the mirror and having formed thereon a drive piezoelectric element. The cantilevers are coupled at one end to the frame with a spring and at a second opposite end to the torsion beams. The spring includes a pair of comb-shaped electrodes 5, 6. When a voltage is applied between the electrodes 5, 6, the resonant frequency of the mirror member 3 changes as a result of a changing spring constant of the torsion beam 2 since a tension force is exerted on the cantilever 4. In the optical scanner disclosed in the above-mentioned Japanese patent application, the cantilevers are connected to the torsion beam, so as to change a spring constant of the torsion beam.
WO2006131916 discloses a MEMS apparatus for scanning an optical beam that comprises a mirror operative to perform a rotational motion to a maximum rotation angle around a mirror rotation axis formed in a double active layer silicon-on-insulator (SOI) substrate. The apparatus includes a bouncing mechanism operative to provide a bouncing event and to reverse the rotational motion. The bouncing event provides the mirror with a piecewise linear response to actuation by intrinsically nonlinear electrostatic forces. An embodiment of the MEMS apparatus includes a mirror, two torsion bars, elevations, and a substrate. Two pairs of short “offset” beams are located in a lower part of the mirror, at an offset b from the top surface. The offset beams are respectively connected by two pairs of C-shaped spring beams to two planar comb drive rotors that have a Y-direction degree of freedom (in the plane of the device and orthogonal to the rotation axis) by their connection to the substrate through retaining beam springs. When a comb drive stator causes an eccentric pull (with eccentricity length b) to its associated spring beam, this yields a rotation moment of the mirror around the torsion bars. The C-shaped spring beams have a nonlinear stiffness designed to transform the movement induced by the comb drives into a linear movement of the mirror (bouncing effect). In the arrangement disclosed by the above-mentioned PCT-application, the spring-beams are used to convert a translatory movement of the actuators into a rotation of the mirror. To that end the spring-beams are attached to the offset beams at an offset b from the top surface. The spring beams are pre-curved within a plane parallel to the plane defined by the frame of the device and have their width in a direction transverse to that plane.